It's logical (to coin a phrase) to conclude that the exodus did not include absolutely every single Vulcan who shared the philosophies of the Sundered. Some would surely have stayed behind, either unwilling to retreat from their homeworld or wishing to wage a guerrilla campaign to win it back.

That's pretty much that I thought, that there was a faction of Romulans who stated on Vulcan, while the majority of them left.
Would make a good novel to explore that history, like a Spock's World kind of novel focusing on the history of the Romulans.